PROBLEM SOLVED.
The issue I was having with Rogue Amoeba Loopback wasn’t due to performance, latency, buffer sizes, or thread priority; the problem was the lack of synchronization. As it would drift with respect to the audio interface, Gig Performer would be forced to wait for samples or skip ahead. In a non-real time recording, you might get one source to drift later and later. In a live setup, we get clicks and pops as things resynchronize.
THE SOLUTION: Create an aggregate device in Audio MIDI Setup that includes the audio interface (with its stable clock) and Loopback, then enable Drift Compensation on Loopback. Now, I can run 64 sample buffers in Gig Performer without artifacts. This results in a round trip of 256 samples, plus overhead, when bringing audio into the interface, through one Loopback, and back out of the interface. It’s rock solid and the latency is 100% acceptable to my ears.
(BTW, thanks for the Blackhole recommendation. When googling about the product and its latency, I stumbled upon an article about Drift Compensation, and the light bulb turned on.)
Now, I can highly recommend Loopback for use with multiple instances of Gig Performer on a Mac.